Tuberculosis (TB) is an especially challenging target for vaccine development, at least in part because Mycobacterium tuberculosis possesses highly effective mechanisms for evading elimination by antigen-specific effector T cells. We recently discovered that one mechanism used by M. tuberculosis to limit the antimycobacterial efficacy of antigen-specific CD4 effector cells is to limit the availability of antigen at the site of infecion in the lungs: when antigens recognized by CD4 effector cells are not available in sufficient quantity, effector T cells are not activated with the frequency needed to eliminate the bacteria. We also discovered that we can overcome the limited availability of antigen in TB by administration of a specific immunodominant epitope peptide, which increases local activation of CD4 effector T cells specific for that epitope, and reduces the bacterial burden in the lungs. In this project, we will use administration of specific epitope peptides from M. tuberculosis to enhance naturally-occurring and vaccine-induced T cell responses in order to bypass antigen (epitope) deficiency. This will reveal the quantitative contribution of limiting antigen availabiliy as a mechanism that impedes TB immunity, and will allow us to characterize other mechanisms that restrict the efficacy of effector T cells in TB. We will first optimize the dosage and interva for epitope peptide administration that optimizes the antimycobacterial activity of CD4 effector T cells, and will determine whether combinations of epitope peptides provide greater antimycobacterial activity without toxicity. We will subsequently examine specific host mechanisms that may limit the antimycobacterial efficacy of epitope peptide administration, to determine the significance of these mechanisms in limiting the efficacy of T cell responses in TB. In addition, we will combine epitope peptide administration with pre-exposure and post-exposure vaccination with state-of- the-art protein subunit TB vaccines, to determine whether pre-exposure vaccination enhances the antimycobacterial effects of epitope peptide administration, and to determine whether epitope peptide administration enhances the antimycobacterial effects of post-exposure vaccination in TB. In additional experiments, we will follow up on our recent observation that administration of a CD4 T cell-specific epitope peptide causes CD4 T cell-dependent activation of CD8 T cells in the lungs of M. tuberculosis-infected mice. We will identify the mechanisms used by CD4 T cells to communicate with CD8 T cells in this context, and we will determine the extent to which those mechanisms contribute to the antimycobacterial activity of CD4 T cells in TB. The proposed studies, which include collaborative studies with an international leader in TB vaccine development, will utilize novel approaches and concepts to provide insight into the mechanisms that limit the efficacy of naturally-occurring and vaccine-induced immunity in TB. In addition, they will provide a basis for development of novel approaches to immunotherapy in TB, by bypassing and modulating specific mechanisms of immune evasion that currently limit the efficacy of TB vaccines.